Deciding the order of SIC detection by adjusting MCS

ABSTRACT

Method, receiver and computer program product for processing a signal, wherein the signal is received over a wireless network from a transmitter, the signal comprising a plurality of data streams. A selected one of the data streams is preferentially treated by reporting an adjusted value of a characteristic of the selected data stream to the transmitter and the received signal is decoded using a technique of successively decoding data streams in which the selected data stream is the first data stream to be decoded, a signal corresponding to the selected data stream being removed from the received signal prior to decoding at least one of the unselected data streams in the received signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the National Stage of, and therefore claims thebenefit of, International Application No. PCT/EP 2011/055662 filed onApr. 12, 2011, entitled “DECIDING THE ORDER OF SIC DETECTION BYADJUSTING MCS,” which was published in English under InternationalPublication Number WO 2011/128312 on Oct. 20, 2011, and has a prioritydate of Apr. 13, 2010, based on GB application 1006103.4. Both of theabove applications are commonly assigned with this National Stageapplication and are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to decoding a signal, and in particular todecoding a signal transmitted via a Multiple-Input Multiple-Output(MIMO) transmission over a wireless network.

BACKGROUND

Recent advances of wireless communications have led to the emergence ofnew multi-user communication techniques, including multi-user diversityand Successive Interference Cancellation (SIC). SIC is the optimalmultiple access scheme to achieve the uplink capacity, see, e.g., D. Tseand P. Viswanath, “Fundamentals of Wireless Communication”, CambridgeUniversity Press, 2005. In a conventional single-decoder receiver, theinterference from data streams associated with other users in thenetwork is treated as noise. Differently, in an uplink receiveremploying SIC, a data stream associated to a first user is decoded andthe corresponding reconstructed data stream is removed from theaggregate received signal, before the next data stream is decoded. Thisprocess is repeated for each data stream until all of the data streamsin the signal have been decoded. SIC techniques can also be used on thedownlink, where data streams intended for different users aresimultaneously transmitted and potentially interfere with each other.Using a SIC receiver at a particular user's device, the data streamswith the highest signal quality (corresponding to the lowest probabilityof decoding error) are decoded first and the corresponding reconstructedsignals are then successively removed from the received signal, beforethe data stream intended for the particular user is decoded. SICprocessing is described for example in M. K. Varanasi and T. Guess,“Optimum Decision Feedback Multiuser Equalization with SuccessiveDecoding Achieves the Total Capacity of the Gaussian Multiple-AccessChannel”, in Proceedings of Thirty-First Asilomar Conference on Signals,Systems, and Computers, vol. 1, pp. 1405-1409, November 1997, and in D.Tse and P. Viswanath, “Fundamentals of Wireless Communication”,Cambridge University Press, 2005. The implementation of a SIC receiverrequires a significant use of processing resources. In the downlink, theuse of SIC techniques at the user equipment (UE) receiver is thereforelimited by its complexity, which scales with the number of users.

In a Multiple-Input Multiple-Output (MIMO) system, spatial multiplexingallows the transmission of multiple data streams (or data layers) overdifferent spatial channels. As is known in the art, multiple transmitantennas can send different data streams over separate spatial channels,and the use of multiple receive antennas can allow the recovery of thedifferent data streams, see, e.g., G. J. Foschini and M. J. Gans, “OnLimits of Wireless Communications in a Fading Environment when UsingMultiple Antennas”, Wireless Personal Communications, vol. 6, no. 3, pp.311-335, March 1998, I. E. Telatar, “Capacity of Multi-Antenna GaussianChannel”, European Transactions of Telecommunications, vol. 10, no. 6,pp. 585-595, November/December 1999, and D. J. Love and R. W. Heath,Jr., “Limited Feedback Unitary Precoding for Spatial MultiplexingSystems”, IEEE Transactions on Information Theory, vol. 51, no. 8,August 2005. As shown schematically in FIG. 1, each transmit antenna 14₁ and 14 ₂ transmits to each (both) receive antennas 16 ₁ and 16 ₂ atthe receiver. Any number of transmit antennas and receive antennas maybe used, and the maximum number of data streams that can bedistinguished due to the spatial multiplexing is equal to the lower ofthe number of transmit antennas and the number of receive antennas.

The transmission system shown in FIG. 1 can be described by the equationr=Hx+n   (1)where r denotes the received signal vector, x is the transmitted signalvector, H indicates the MIMO channel matrix, and n is the noise(noise-plus-interference) vector. The channel matrix H models thecharacteristics of the MIMO propagation channel. In the case of afrequency non-selective channel, Equation (1) can be expanded as

$\begin{matrix}{{\begin{bmatrix}r_{1} \\r_{2}\end{bmatrix} = {{\begin{bmatrix}h_{11} & h_{12} \\h_{21} & h_{22}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2}\end{bmatrix}}},} & (2)\end{matrix}$where r₁ and r₂ are the signals received at the respective receiveantennas 16 ₁ and 16 ₂; x₁ and x₂ are the signals transmitted from therespective transmit antennas 14 ₁ and 14 ₂; h₁₁, h₁₂, h₂₁ and h₂₂ arethe coefficients of the (frequency non-selective) MIMO channel; and n₁and n₂ are the noise (noise-plus-interference) at the respective receiveantennas 16 ₁ and 16 ₂. The noise-plus-interference term typicallyincludes noise generated inside the receiver, which is conventionallymodelled by an equivalent stochastic process at the antenna connector.

A successive interference cancellation (SIC) technique can be used toimprove the decoding process in the MIMO system. Once a data stream iscorrectly decoded, the corresponding received signal component can bereconstructed and subtracted from the received signal. Assuming thateach data stream is decoded perfectly and the correspondingreconstructed signal is subtracted at each stage of the SIC procedure,then decoding the final data stream is performed with no interferencecaused by the other data streams in the received signal.

The SIC technique is applicable to both cases of Multi-User MIMO(MU-MIMO) transmission, where the multiple spatially separated datastreams are simultaneously transmitted to multiple users, andSingle-User MIMO (SU-MIMO) transmission, where the multiple paralleldata streams are transmitted to a single user.

The SIC technique is particularly sensitive to error propagation. If anerror occurs in decoding or subtracting one of the data streams, thenthe subsequent data streams to be processed are likely to be affected byan increased probability of decoding errors. The ordering of the datastreams can affect the error propagation. In a MIMO system where all ofthe data streams are equally protected by channel coding, the datastreams are decoded starting from the strongest data stream (in a Signalto Noise Ratio sense or based on other suitable metrics) and continuingby decoding progressively weaker data streams in sequence.

SUMMARY

The present invention relates to improvements in the decoding of signalstransmitted via a MIMO transmission. In particular, the case where themodulation scheme and code rate of the different data streams/layers areindividually adapted to the channel conditions based on feedback of thecurrent channel quality is considered.

According to a first aspect of the invention there is provided a methodof processing a signal, the method comprising: receiving the signal overa wireless network from a transmitter, the signal comprising a pluralityof data streams; preferentially treating a selected one of the datastreams by reporting an adjusted value of a characteristic of theselected data stream to the transmitter; and decoding the receivedsignal using a technique of successively decoding data streams in whichthe selected data stream is the first data stream to be decoded, asignal corresponding to the selected data stream being removed from thereceived signal prior to decoding at least one of the unselected datastreams in the received signal.

According to a second aspect of the invention there is provided areceiver for processing a signal, the receiver comprising: receivingmeans for receiving the signal over a wireless network from atransmitter, the signal comprising a plurality of data streams; meansfor preferentially treating a selected one of the data streams byreporting an adjusted value of a characteristic of the selected datastream to the transmitter; and decoding means for decoding the receivedsignal using a technique of successively decoding data streams in whichthe selected data stream is the first data stream to be decoded, thedecoding means being configured to remove a signal corresponding to theselected data stream from the received signal prior to decoding at leastone of the unselected data streams in the received signal.

According to a third aspect of the invention there is provided awireless network comprising: a transmitter for transmitting a signalover the wireless network; and a receiver as described above in thesecond aspect of the invention for receiving and processing the signal.

According to a fourth aspect of the invention there is provided acomputer program product comprising computer readable instructions forexecution at a receiver of a wireless network, the receiver beingconfigured to receive a signal transmitted over the wireless networkfrom a transmitter, the signal comprising a plurality of data streams,the instructions comprising instructions for: preferentially treating aselected one of the data streams by reporting an adjusted value of acharacteristic of the selected data stream to the transmitter; anddecoding the received signal using a technique of successively decodingdata streams in which the selected data stream is the first data streamto be decoded, a signal corresponding to the selected data stream beingremoved from the received signal prior to decoding at least one of theunselected data streams in the received signal.

Embodiments of the present invention address the problem of ordering thedata streams in a received MIMO signal for use in a SIC decodingtechnique. Typically, the modulation scheme and coding rate of thedifferent data streams transmitted in the downlink are individuallyadapted to the channel conditions based on feedback of the currentchannel quality from the receiver.

Error propagation is minimized by sending appropriate information on thequality of the data streams from the receiver to the transmitter. In aSingle-User MIMO (SU-MIMO) system, i.e., a MIMO system where all of theparallel data streams are intended for a single receiver, the receivercan preferentially treat one of the data streams such that it is chosento be decoded first in the SIC decoding. The channel quality of thepreferentially treated data stream is underreported to the transmitter(e.g. by reporting an adjusted value of the channel quality). Inresponse the transmitter will follow the reported channel quality forthat particular data stream, which will in turn reduce the error-rate ofthat particular data stream after decoding. Since that data stream ischosen to be the first data stream to be decoded in the SIC decoding,fewer errors propagate through to the decoding of the other datastreams. In this way, the performance of the overall SIC decoding willbe improved.

Information on the quality/reliability of each data stream (e.g. thecoding rate and modulation scheme of each data stream) can also be usedto adaptively switch between a non-SIC decoding technique and a SICdecoding technique. When decoding one of the data streams in thereceived signal, the non-SIC decoding technique treats the other datastreams as noise. Differently, as described above, the SIC decodingtechnique decodes one data stream and then removes the reconstructedsignal corresponding to that data stream from the received signal beforedecoding subsequent data streams. Compared to the non-SIC decodingtechnique, the SIC decoding technique may result in improved performance(increased supported data rate or reduced error rate), but requires moreprocessing power to implement. Depending on the conditions on the datastreams and/or on the capabilities of the receiver, the receiver candynamically choose between using the SIC decoding technique or thenon-SIC decoding technique.

The information on the channel quality for each data stream (e.g., thereported coding rate and modulation scheme of each data stream) can alsobe used to decide the number of data streams that can be supported inthe MIMO transmission. This can be reported from the receiver to thetransmitter using a rank indicator (RI), which indicates the rank of thechannel matrix. The rank of the channel matrix is defined as the numberof linearly independent columns of the channel matrix and can be used toindicate the number of spatial dimensions of the MIMO channel. Forexample, for a MIMO transmission with four transmit antennas (N_(T)=4)and four receive antennas (N_(R)=4), the 4×4 channel matrix can haverank equal to 4, 3, 2 or 1 (rank≦min (N_(T), N_(R))). The rank of thechannel matrix also determines the size of a precoding matrix which isused by the transmitter, i.e., the number of columns of the precodingmatrix.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be put into effect, reference will now be made, by way ofexample, to the following drawings in which:

FIG. 1 is a schematic representation of MIMO transmission with N_(T)=2transmit antennas and N_(R)=2 receive antennas;

FIG. 2 is a schematic representation of a transmitter and a receiveraccording to a preferred embodiment;

FIG. 3 a is a flow chart for a process of decoding a signal according toa SIC technique;

FIG. 3 b is a flow chart for a process of decoding a signal according toa first method;

FIG. 4 is a flow chart for a process of decoding a signal according to asecond method; and

FIG. 5 is a flow chart for a process of decoding a signal according to athird method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIG. 2 there is now described a preferred embodimentof the present invention. A transmitter 202 comprises a transmit antennablock 20 having multiple antennas, such as antennas 14 ₁ and 14 ₂ shownin FIG. 1, for use in MIMO transmission. A receiver 206 comprises areceive antenna block 208 having multiple antennas, such as antennas 16₁ and 16 ₂ shown in FIG. 1, for use in MIMO transmission. The schematicrepresentation of FIG. 1 refers to the case of N_(T)=2 transmit antennasand N_(R)=2 receive antennas only for simplicity of illustration, whilethe validity of the present invention is maintained for a differentnumber of transmit and receive antennas. The receiver 206 also comprisesprocessing means 210 and a user interface 212. The processing means 210is connected to the receive antenna block 208 and to the user interface212. The user interface 212 is used to communicate data to and from auser of the receiver 206. For example, the user interface may comprise aspeaker and a visual display for outputting data to the user and amicrophone, a camera and a keypad allowing the user to input data to thereceiver 206.

In operation, data is transmitted wirelessly from the transmitter 202 tothe receiver 206 with a MIMO transmission using the transmit antennablock 204 and the receive antenna block 208. In this way multiple datastreams are transmitted between the transmitter 202 and the receiver206. A Successive Interference Cancellation (SIC) technique is used todecode the data streams at the receiver 206. In order to minimize errorpropagation in the SIC technique, it is advantageous to decode a fulldata stream before cancellation of the reconstructed signalcorresponding to the considered data stream from the received signal. Itis preferable to decode the different data streams in an order thatmatches their quality/reliability in the received signal. Morespecifically, the receiver 206 is adapted to first decode the mostreliable data stream, where the most reliable data stream is the oneproducing the lowest number of errors at the decoder output. In thisway, the error propagation is minimized in the SIC technique.

With reference to FIG. 3 a, there is now described a method of decodinga signal according to a SIC technique. In step S302 the signal isreceived at the receive antenna block 208. The signal comprises multipledata streams transmitted as described above from the transmit antennablock 204.

In step S304 the data streams are analysed to determine an ordering fordecoding the data streams and then in step S306 the data streams areordered in accordance with the analysis performed in step S304. In stepS308 the first data stream of the ordered data streams is selected anddecoded.

In step S310 the remaining data streams in the signal are decoded usinga SIC technique. In this way, the ordering performed in steps S304 andS306 determines which data stream is decoded first. Once the first datastream has been decoded, the reconstructed signal corresponding to thefirst data stream is removed from the received signal. The SIC techniquecontinues by successively decoding the remaining data streams inaccordance with the ordering established in step S306.

Following the analysis of the data streams in step S304 the method alsopasses to step S312 in which the quality of the data streams iscomputed. The quality of the data streams is reported to the transmitterin step S314.

With reference to FIG. 3 b, there is now described a method of decodinga signal according to a preferred embodiment. In step S322 the signal isreceived at the receive antenna block 208. The signal comprises multipledata streams transmitted as described above from the transmit antennablock 204.

In step S324 the data streams are analysed. However, in contrast to theanalysis performed in step S304 of FIG. 3 a, an ordering for decodingthe data streams is not determined.

Following step S324 the method passes to step S326 in which the qualityof the data streams is computed. In step S328 a data stream is selectedand the value of the quality of the selected data stream to be reportedto the transmitter is adjusted. In step S328 more than one data streammay be selected and the values of the quality of the selected datastreams to be reported to the transmitter may be adjusted. It can beparticularly useful to select more than one data stream in step S328when there are more than two data streams in total. In step S330 thereceiver 206 then sends a report of the quality of the data streams tothe transmitter 202, which includes the adjusted quality value for theselected data stream(s). Each data stream is encoded separately at thetransmitter 202 such that each data stream is coded and mapped to anumber of modulation symbols by the transmitter 202 depending on theinformation on the reception quality fed back by the receiver 206 oneach data stream. The information describes the channel quality and itis often reported in terms of a corresponding Modulation and CodingScheme (MCS).

The analysis of the data streams performed in steps S324 and S326 isimplemented in software in the preferred embodiment in the processingmeans 210. In alternative embodiments the analysis is carried out inhardware rather than in software.

In steps S332 and S334 the signal is decoded using a SIC technique. Inthis way, the data stream selected in step S328 is decoded first in stepS332. Once the selected data stream has been decoded, the reconstructedsignal corresponding to the selected data stream is removed from thereceived signal. The SIC technique continues by decoding the remainingdata streams. In this way, the number of errors in the decoded signal,across all the data streams, is reduced. Where more than one data streamis selected in step S328 those selected data streams are decoded beforethe remaining data streams are decoded.

Step S332 uses the information on which data stream has had an adjustedquality value reported to the transmitter 202 in previous TransmissionTime Interval(s) (TTI). This means that if in step S328 in a particularTTI a new data stream is selected for reporting an adjusted value of thequality to the transmitter, then step S332 will not switch to decodingthat new data stream first in that particular TTI. Only in subsequentTTIs will step S332 switch to decoding the new data stream first.

In the preferred embodiment, in the case of Single-User MIMO (SU-MIMO)transmission, the receiver 206 determines that a particular data streamwill be decoded first in the SIC decoding. Furthermore, the receiver 206reports a lower MCS to the transmitter 202 in step S330 for theparticular data stream. In other words, the receiver 206 reports to thetransmitter 202 that one of the data streams is received with a lowerquality than it actually is. This is referred to as ‘underreporting’ thequality of the data stream. In this way, the receiver reports anadjusted value of a characteristic (in this case the quality) of aparticular data stream to the transmitter. In response to receiving thisreport, the transmitter 202 will transmit a more robust MCS (requiring alower Signal-to-Noise Ratio for a given error probability), which inturn will decrease the probability of error for the considered datastream, when decoded by the receiver 206. The information of the bias inthe reported channel quality of the particular data stream will be takeninto account in the ordering and selection of the data streams in stepS332. In this way the receiver 206 will in practice determine theordering of the data streams in the SIC decoding. Furthermore, byunderreporting the quality of the first data stream to be decoded, theblock error rate of that particular data stream is reduced, such thatthe effect of error propagation in the SIC decoding is reduced. In thisway the overall performance of the receiver 206 is improved. In fact,any errors in decoding the data stream which is selected to be decodedfirst in the SIC decoding technique would propagate through to thedecoding of all of the other data streams, and it is thereforeparticularly important to reduce the decoding errors on the first datastream to be decoded. The approach can be extended to the reporting andordering of the remaining data streams.

This scheme is particularly useful in the case where the data streamsexperience similar propagation conditions. In this case, all the datastreams will have a similar probability of error, which in turn resultsin a lack of ordering of the reception quality of the data streams atthe receiver 206. This lack of ordering will cause a degradation of theperformance of a conventional SIC decoding technique. In the preferredembodiment described above, an adjusted value of the channel quality ofa particular data stream is reported to the transmitter 202, and thereceiver chooses the particular data stream to be the first to bedecoded and subtracted from the received signal. In this way, theoverall performance of the receiver is improved by increasing thereliability of SIC processing.

In conventional SIC decoding techniques the first step is often toanalyse the data streams to thereby select the data stream which has thehighest quality to be the first data stream to be decoded. However, inpreferred embodiments of the present invention the SIC decoding does notrequire a step of analysing the data streams to thereby order the datastreams (i.e. a step similar to step S306 of FIG. 3 a is not required inthe method described above in relation to FIG. 3 b). Instead aparticular data stream is chosen by the receiver to be the first datastream to be decoded, and the quality (or some other characteristic,e.g. the reliability) of the chosen data stream is underreported to thetransmitter, such that the chosen data stream is preferentially treatedto have an improved quality (or an improved characteristic, e.g.reliability). This removes the need for the analysing and selectingsteps in the SIC decoding which can reduce the processing required toperform the SIC decoding.

The data stream chosen to be decoded first in the SIC decoding and to beunderreported to the transmitter can be chosen based on the quality ofthe received data stream. For example, the data stream may be chosenbecause it has the highest quality of the received data streams. Thedata stream may be chosen because it has the lowest quality of the datastreams, hence reducing the complexity of encoding and subtracting thatparticular stream. Alternatively, the data stream can be chosenindependently of the quality of the data streams, e.g. the data streamcan be chosen randomly. It will be apparent to the skilled person thatdifferent methods for choosing a data stream may be employed indifferent embodiments of the invention.

The receiver could additionally use the information provided by thecyclic redundancy check (CRC) of each decoded block of the first datastream, and decide depending on the CRC which block is to be regeneratedand subtracted from the received signal. That is, the technique works atthe block level but also at the codeword level. One transport block ofone stream may contain multiple coded blocks each with its own CRC.

In the preferred embodiment described above the ordering of the datastreams depends on the reports generated at the receiver 206 and on thechannel quality. This is advantageous in that the receiver 206 hascontrol over the ordering of the data streams.

With reference to FIG. 4 there is now described a second method ofdecoding a signal. Steps S402 and S404 correspond to steps S312 and S314described above. Therefore in step S402 a signal comprising multipledata streams is received at the receive antenna block 208 and in stepS404 the data streams are analysed to determine information on thereceived quality (supported MCS) of each data stream. The information onsupported channel quality/reliability and the MCS for each data streamis used to adaptively switch between a conventional decoding techniqueand a SIC decoding technique. In step S406 a SIC decoding technique or aconventional decoding technique is selected. As described above, the SICtechnique involves successively decoding one of the data streams andthen removing the data stream from the received signal before decodingsubsequent data streams. In this way the data streams are decoded inorder and the interference caused by a data stream is reduced whendecoding a subsequent data stream. However, in a conventional decodingtechnique (i.e. a non-SIC decoding technique) each data stream isdecoded from the received signal separately and the other data streamsare treated as noise in the received signal. If the analysis of stepS404 suggests that all the data streams experience communicationchannels with similar quality/reliability, and specifically with similarprobability of error, then the ordering of the data streams isdifficult, and the improvement in using a SIC technique as compared tousing a conventional decoding technique would be limited. In that case,in order to save in complexity, in step S406 a conventional decodingtechnique would be selected because a conventional decoding techniqueuses less of the processing resources of the receiver 206 than would beused by a SIC decoding technique. The selection in step S406 may bebased, at least in part, on the processing resources available at thereceiver 206.

In step S408 the decoding technique selected in step S406 is used todecode the received signal, and the final step is to report the qualityof the data streams to the transmitter in step S410.

In this way, where the use of a SIC decoding technique is advantageous,the SIC decoding technique is selected in step S406. However, where thedetriment of the extra processing resources required to perform the SICdecoding technique outweighs the advantages of the improved quality ofdecoded data streams produced by using the SIC decoding technique, thena conventional decoding technique is used instead of the SIC decodingtechnique. This provides the receiver 206 with added flexibility foradapting the decoding technique to the specific conditions on thecommunication channel. The decision between a SIC decoding technique anda conventional decoding technique can be made dynamically in response tochanging channel conditions.

By way of example, the case of two data streams is now considered. Thereception quality of the data streams in the received signal isdetermined, and the difference in the probability of error in decodingthe two data streams is estimated. If the difference in the estimatederror probability of the two data streams is greater than a thresholdvalue, then the SIC decoding technique is used, because it isadvantageous to remove the more reliable data stream before decoding theless reliable data stream. However, if the difference in the estimatederror probability of the two data streams is less than the thresholdvalue, then the conventional decoding technique is used, because it isadvantageous to avoid the extra processing required by a SIC decodingtechnique.

In pseudo-code this example can be described as follows:

if (Delta_P_(e) > θ) use MMSE-SIC else use MMSE endwhere Delta_P_(e) denotes the difference between the estimated errorprobability of the second data stream and the estimated errorprobability of the first data stream, θ is a design parameter indicatinga threshold value, MMSE-SIC is a minimum mean square error SIC decodingtechnique and MMSE is a conventional minimum mean square error decodingtechnique.

In a third method of decoding a signal the CQI information is used toindicate the rank of the MIMO communication channel, i.e. the number ofdata streams that can be supported on the communication channel. Theactual value of the CQI of the first data stream and the differenceDelta_CQI between the reported Channel Quality Indicator (CQI) of thetwo data streams, are used by the receiver 206 to form an indication ofthe condition of the communication channel. This indication can be usedto determine the number of independent data streams, i.e., the rank ofthe communication channel. The Delta_CQI gives a relative measure of thequality of the two data streams in contrast to the actual value of theCQI which gives an independent measure of the quality of a data stream.It is also possible to use the actual value of the CQI for the first andsecond data streams rather than using the actual value of the CQI of thefirst data stream and the difference Delta_CQI between the reportedChannel Quality Indicator (CQI) of the two data streams. The receiverand the transmitter use the same scheme for defining the CQIs of thedata streams.

The third method is described with reference to FIG. 5. Similarly, tosteps S302 and S304, in step S502 the signal is received at the receiver206 from the transmitter 202 via a MIMO transmission. In step S504 thereceived signal is analysed to determine the quality of the datastreams. In this way a Channel Quality Indicator (CQI) is produced foreach of the data streams.

In step S506 the rank of the channel is determined at the receiver 206.The number of data streams that can be supported in a MIMO transmissionis linked to the condition number of the channel matrix H (i.e. the rankof the channel matrix H). The rank of the channel matrix provides thenumber of independent data streams that can be supported in the MIMOtransmission. A channel matrix of full rank will provide the maximumnumber of independent data streams, whereas an ill-conditioned channelmatrix will give a lower number of independent data streams.

As an example, for two transmit antennas and two receive antennas (asshown in FIG. 1) a channel matrix of

$H = \begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}$has a rank of two because there are two independent columns. This meansthat two independent data streams can be used in the MIMO transmission.In fact, with this channel matrix, the data stream transmitted from eachtransmit antenna can be recovered independently.

However, a channel matrix of

$H = \begin{bmatrix}1 & 1 \\1 & 1\end{bmatrix}$has a rank of one because the columns are not independent. This meansthat only one independent data stream can be used in the MIMOtransmission.

The channel matrix varies dynamically as the conditions of the channelvary. Therefore the rank of the channel matrix is determined dynamicallyin order to optimize the number of independent data streams inaccordance with the current conditions for the MIMO transmission.

When the rank of the channel matrix has been determined, in step S508 aRank Indicator (RI) message is reported from the receiver 206 to thetransmitter 202 to inform the transmitter 202 of the determined channelrank to be used on the communication channel.

As another example, in the case of two transmit antennas and two receiveantennas (as shown in FIG. 1), a situation where the Signal-to-NoiseRatio (SNR) is relatively low and the CQI difference Delta_CQI isrelatively high, would indicate that there is one strong data stream andone weak data stream. In these conditions, the communication channelcannot realistically support spatial multiplexing of two streams, and sothe message sent to the transmitter 202 instructs the transmitter 202 touse a channel with a rank of one.

Alternatively, the third method of decoding a signal may be implementedat the transmitter after having received a channel quality indicator foreach of the data streams from the receiver. In this case, thetransmitter determines the number of independent data streams that canbe supported in the MIMO transmission of the signal based on the channelquality indicator received from the receiver. The transmitter can thentransmit the signal to the receiver using the determined number ofindependent data streams.

There are described above new methods of decoding a received signal in aMIMO system. The methods described above introduce an adaptationcriterion for SIC processing; simplify the data stream orderingoperation; improve the system performance; provide a solution for thecase where all the data streams are experiencing similarquality/reliability; do not require any further processing with respectto the implementation of the SIC receiver; and do not impact otherend-users, in contrast to power-controlled systems, where by changingthe power level one may modify the interference level or decrease thereliability.

In the methods described above, the functional steps shown in FIGS. 3 to5 are performed in software by the processing means 210. In alternativeembodiments, these steps are performed by hardware in the receiver 206.

While this invention has been particularly shown and described withreference to preferred embodiments, it will be understood to thoseskilled in the art that various changes in form and detail may be madewithout departing from the scope of the invention as defined by theappendant claims.

The invention claimed is:
 1. A method of processing a signal, the methodcomprising: receiving the signal over a wireless network from atransmitter, the signal comprising a plurality of data streams;preferentially treating a selected one of the data streams by reportingan adjusted value of a characteristic of the selected data stream to thetransmitter; and decoding the received signal using a technique ofsuccessively decoding data streams in which the selected data stream isthe first data stream to be decoded, a signal corresponding to theselected data stream being removed from the received signal prior todecoding at least one of the unselected data streams in the receivedsignal.
 2. The method of claim 1 wherein the selected data stream hasthe highest quality of the plurality of data streams.
 3. The method ofclaim 1 wherein the selected data stream has the lowest quality of theplurality of data streams.
 4. The method of claim 1 wherein the selecteddata stream is selected independently of the quality of the plurality ofdata streams.
 5. The method of claim 1 wherein the signal is transmittedfrom a plurality of spatially separated transmit antennas using aMultiple-Input Multiple-Output transmission, and wherein the step ofreceiving the signal comprises receiving the signal at a plurality ofspatially separated receive antennas.
 6. The method of claim 1 furthercomprising analysing the received signal to dynamically determine thecharacteristic of the selected data stream in the received signal. 7.The method of claim 1 further comprising: reporting to the transmitteran unadjusted value of the characteristic of a respective at least oneunselected data stream in the received signal.
 8. The method of claim 1further comprising analysing the received signal to dynamicallydetermine the characteristic of each of the plurality of data streams inthe received signal.
 9. The method of claim 8 wherein the adjusted valueof the characteristic of the selected data stream reported to thetransmitter indicates a quality that is lower than the determinedquality of the selected data stream.
 10. The method of claim 1 whereinthe reported characteristic describes a Modulation and Coding Scheme tobe used by the transmitter for each of the data streams.
 11. The methodof claim 1 wherein the characteristic is a Channel Quality Indicator fora respective data stream.
 12. The method of claim 1 wherein thecharacteristic comprises the decoding error probability in therespective received data stream.
 13. The method of claim 1 wherein thecharacteristic is determined by performing at least one CyclicRedundancy Check algorithm on each respective data stream in thereceived signal.
 14. The method of claim 1 wherein the signalcorresponding to the selected data stream is removed from the receivedsignal by subtraction.
 15. A receiver for processing a signal, thereceiver comprising: receiving means for receiving the signal over awireless network from a transmitter, the signal comprising a pluralityof data streams; means for preferentially treating a selected one of thedata streams by reporting an adjusted value of a characteristic of theselected data stream to the transmitter; and decoding means for decodingthe received signal using a technique of successively decoding datastreams in which the selected data stream is the first data stream to bedecoded, the decoding means being configured to remove a signalcorresponding to the selected data stream from the received signal priorto decoding at least one of the unselected data streams in the receivedsignal.
 16. The receiver of claim 15 wherein the receiving meanscomprises a plurality of spatially separated receive antennas forreceiving the signal, the signal being transmitted from a plurality ofspatially separated transmit antennas using a Multiple-InputMultiple-Output transmission.
 17. The receiver of claim 15 furthercomprising means for analysing the received signal to determine thecharacteristic of the data streams.
 18. A wireless network comprising: atransmitter for transmitting a signal over the wireless network; and areceiver according to claim 15 for receiving and processing the signal.19. A computer program product comprising computer readable instructionsstored on a non-transitory computer readable medium for execution at areceiver of a wireless network, the receiver being configured to receivea signal transmitted over the wireless network from a transmitter, thesignal comprising a plurality of data streams, the instructionsconfigured to direct the operation of a processor when executed to:preferentially treat a selected one of the data streams by reporting anadjusted value of a characteristic of the selected data stream to thetransmitter; and decode the received signal using a technique ofsuccessively decoding data streams in which the selected data stream isthe first data stream to be decoded, a signal corresponding to theselected data stream being removed from the received signal prior todecoding at least one of the unselected data streams in the receivedsignal.